This invention relates to thermal tempering, or "heat strengthening," glass sheets or the like, and in particular to the quenching stage of the tempering process, where cooling blasts of gaseous tempering medium (usually air) are directed onto heated sheets of glass to quickly reduce the temperature of the surface portions of the sheets. More specifically, the invention is concerned with the type of quenching arrangement wherein the glass sheets are treated while in a generally horizontal or oblique orientation and their lower major surfaces are supported out of contact with solid structures by means of the fluid pressure of gaseous quenching blasts. Examples of gas-support quenches for tempering glass sheets may be seen in U.S. Pat. No. 3,223,501 to Fredley et al. and in U.S. Pat. No. 3,332,759 to McMaster et al.
Gas-support quenches can usually be operated with a high degree of reliability. However, due to the high stresses that are created in the glass by the tempering process, defects in the glass or non-uniformities or imbalances in the heating or cooling of the glass sheets can occasionally lead to glass breakage in the quench station. When breakage occurs, it is imperative that the broken glass fragments be removed immediately from the quench station before the next sheet of glass conveyed into the quench station collides with the stationary fragements to create a jam-up. If the next sheet comes into contact with the fragments, it too is likely to break, making the breakage problem more severe, or at least the glass sheet's surface may become marred. Not only is production lost by such a jam-up, but the accumulation of a large number of small glass fragments can clog the air passages, or cause a fragment to become lodged in a location where it may contact and mar several glass sheets subsequently conveyed through the quench station.
It is customary for operators to remove glass fragments from the quench with a hand-held rod or rake-like device which is thrust repeatedly between the upper and lower arrays of quench nozzles to knock the glass fragments out of the apparatus. But because the space between the upper and lower quench nozzles is relatively narrow, it is often difficult to complete the removal within the short time available (usually just a few seconds) before the next sheet enters the quench. Additionally, an operator's haste in attempting to quickly remove the fragments from the narrow space can sometimes cause damage to the quench nozzles.
A quench station usually has some provision for raising the upper nozzle section to gain better access to the interior, but it is generally inadvisable to do so during production to remove glass fragments. This is due to the fact that opening the quench eliminates the back-pressure effect of the opposed upper and lower arrays of nozzles, and the rate of flow is thereby increased, which has a number of undesirable consequences. One such consequence is that an unopposed flow from the lower nozzles can blow glass fragments violently out of the opened section. This result is not only hazardous to the operators, but can propel fragments into other sections of the tempering line where they may cause damage to the glass. Another drawback is that the increased air flow sometimes sends relatively cool currents of air upstream toward or into the heating chamber. As a result, glass sheets leaving the furnace at that time are heated non-uniformly or heated insufficiently for proper tempering, with the result that additional breakage may occur in the quench station or some of the glass sheets may not attain the desired temper.
Shutting off the blowers that supply the pressurized air to the quench nozzles is not a practical solution to the aforesaid problems. the large blowers which are required to supply air under pressure to the quench station need several minutes to come to a stop after they have been shut off. But since broken glass fragments must be removed within a few seconds to avoid subsequent sheets colliding with the fragments, such a delay would not be acceptable.